In a large saucepan over medium heat, heat the oil, and add the onion; sauté 5 minutes. Add the garlic and sauté until the onion is tender, about 5 minutes. Add the crushed red pepper and sauté 30 seconds. Add the tomatoes. Simmer until the sauce is reduced to 5 cups, stirring occasionally, about 1 hour. Season to taste with salt and pepper. Remove from heat and mix in the fresh basil.: The moment the extra virgin olive oil warms and moves like liquid glass is when you know the pan is ready, and adding the onion produces a soft sizzling sound as the edges turn translucent. This gentle sauté releases sweet aromatic compounds that form the flavor base of the sauce, and taking five minutes to coax the onion into softness avoids harsh raw notes. Watch the color closely, because if the onion begins to brown rapidly you have the heat too high and will risk developing bitter flavors. Stir occasionally so each piece softens evenly, and if pieces start to stick add a splash of water to deglaze and prevent scorching.
In a medium bowl, combine the ricotta, 1 1/3 cups of the feta cheese, basil and chives. Season to taste with salt and pepper. Stir in the eggs.: As you stir in the minced garlic , inhale the immediate burst of warmth that signals the aromatics are coming together. The garlic softens and melds rather than standing alone, which deepens the savory backbone of the sauce. This step is about timing more than temperature; keep the heat moderate to prevent the garlic from turning brown and bitter. If you detect an acrid smell, remove the pan from heat briefly to cool and then continue, since burnt garlic is hard to rescue.
Cook the shells in large pot of boiling salted water until just tender but still firm to bite. Drain. Rinse with cold water until cool. Drain thoroughly.: The momentary addition of crushed red pepper releases a background of gentle heat, and you should sense a faint lift in the aroma within seconds. Sautéing for just thirty seconds blooms the pepper without overpowering, letting it infuse the oil and aromatics. Overcooking here can amplify heat and overshadow the herbs and cheeses, so be watchful and move on to the next step promptly.
Preheat oven to 350 degrees F.: When you pour in the canned tomatoes you will hear a gentle bubbling that slowly evolves into a steady simmer, and over the course of about an hour the sauce will thicken and darken as excess water evaporates. The simmer concentrates sweetness and acidity, producing a sauce that clings to the shells rather than sliding off. Stir occasionally to prevent sticking, and note the visual cue of a glossy, slightly reduced surface; when it measures roughly five cups in volume the flavors will be nicely concentrated. Seasoning with salt and pepper is essential to balance the acidity, and folding in the fresh basil at the end preserves its green perfume. A common mistake is rushing the reduction, which leaves a watery sauce that fails to coat the pasta shells .
Spread 3/4 cup of sauce in the bottom of each of two 13×9-inch glass baking dishes. Fill 30 shells and divide between the dishes. Top with the remaining sauce and sprinkle with remaining feta.: The filling comes alive when you mix the creamy ricotta cheese with the salty, crumbly feta cheese , bright chopped basil , and verdant chives . As you fold in the eggs , you will notice the mixture take on a silkier, more cohesive texture that will firm up during baking, so gentle folding is key to retain creaminess. Season carefully because the feta cheese already contributes saltiness; taste before adding more salt to avoid oversalting. Overmixing can break down the cheese structure and make the filling runny, so stop when the ingredients are evenly combined and still slightly lumpy.
Bake until the shells are heated through, about 40 minutes. Garnish with additional basil leaves.: Bring a large pot of water to a vigorous boil, add enough salt so it tastes like the sea, and cook the jumbo pasta shells until they are al dente, where they yield to pressure but retain a slight resistance. The boiling water should produce steady rolling bubbles and a lively sound; timing matters because overcooked shells become fragile and can tear when stuffed. Immediately rinse them with cold water to stop cooking and to make them cool enough to handle; this step also prevents sticking. Drain thoroughly so excess water does not dilute the filling or make the dish watery when assembled. A frequent error is underdraining, which leads to a soggy casserole.
Preheat oven to 350 degrees F: Preheating the oven ensures even baking and that the filling sets properly once the shells go in. When you set the oven to 350 degrees F , allow ample time for it to reach temperature so the initial bake isn’t compromised by a cold start. A reliable oven thermometer helps verify actual temperature, because inaccurate heat can result in underbaked centers or overly browned tops. Avoid opening the oven repeatedly during the first part of baking, as this causes temperature fluctuations that affect the texture.
Spread 3/4 cup of sauce in the bottom of each of two 13×9-inch glass baking dishes fill 30 shells and divide between the dishes top with the remaining sauce and sprinkle with remaining feta: When you spread the sauce into the bottom of the baking dishes it prevents sticking and creates a flavorful bed for the shells, and the gloss of the sauce is a visual cue that it is properly reduced. Fill each cooled pasta shell with the cheese mixture so the interior is creamy and slightly domed, then nestle them into the sauced dish. Spoon the remaining sauce over the shells so each one is covered, and scatter the reserved feta cheese on top for salty, golden pockets after baking. Take care not to overcrowd the pans, and if a shell cracks during filling tuck it seam side down so the filling stays put. Overfilling shells can cause the filling to spill and burn at the edges.
Bake until the shells are heated through about 40 minutes garnish with additional basil leaves: As the casserole bakes you will notice a gentle bubbling sound and an aroma of roasted tomatoes and warmed cheeses rising from the oven, and the surface will take on a slight browning where the feta cheese crisps. The eggs in the filling will set so the interior becomes sliceable but still creamy. Check for hot bubbling and a warm center to ensure doneness; if the edges brown too quickly, tent with foil to prevent burning. Garnish with extra fresh basil leaves for a final green perfume before serving.